An LED is a semiconductor diode, converting electrical energy into light energy, and emitting visible (e.g. yellow, green and blue), infrared or ultraviolet light. Compared with incandescent light bulbs and neon lamps, LEDs have the advantages including lower working voltage and current, higher reliability, longer lifetime and greater intensity flexibility.
The first LED using liquid-phase epitaxy was a red LED. Since Metal Organic Chemical Vapor Deposition (MOCVD) was invented, aluminium gallium indium phosphide (AlGaInP) has been widely used to make high-power high-intensity red and yellow LEDs. Although AlGaInP-based red LEDs are commercially available, their light-emission efficiency is low, because the gallium arsenide (GaAs) substrate absorbs red light and the angle of emergence is small due to the refractive index of the semiconductor material. Therefore, with the commercial manufacturing, studies are still being conducted to improve the light-emission efficiency of AlGaInP red LEDs.
Nowadays, major techniques for improving the light-emission efficiency of red LEDs include: thickening the gallium phosphide (GaP) window layer, growing a distributed Bragg reflector (DBR) before the red light-absorbing GaAs substrate, replacing the red light-absorbing GaAs substrate with a red light-transparent GaP layer, and using flip-chip structure with a reflective metal layer.
Chinese patent application CN1373522A discloses an LED with a substrate coated with a reflective metal film and a fabrication method thereof. The LED has a permanent substrate that has a reflective metal surface. The method includes forming an LED assembly on a temporary substrate, adhering the LED assembly to a substrate with a reflective layer (i.e., the permanent substrate), and then removing the temporary substrate that absorbs light, so that light emitted by the LED assembly is no longer absorbed by the substrate and light towards the substrate is now reflected from the surface, resulting in higher intensity. However, the reflection of light by the metal in the invention is limited, normally less than 90%. Moreover, in the single reflective metal layer structure, the metal comes into direct contact with the semiconductor, which may form an alloy at a certain temperature as a result of diffusion and lead to significant drop in the reflective index of the metal layer.
Chinese patent application CN1897316A discloses a high-intensity LED structure with a reflective layer, which includes: a substrate, and a metal layer, an unalloy ohmic contact layer and a light-emitting structure which are formed on the substrate in that order. The metal layer is used as a reflective layer, and is made of pure metals or a metal nitride, to obtain good reflectivity. The unalloy ohmic contact layer is arranged between the metal layer and the light-emitting structure, providing a desired ohmic contact. To keep the metal layer clear from the unalloy ohmic contact layer, and to maintain flatness of the reflective surface of the first metal layer, an optional dielectric layer is arranged between the metal layer and the unalloy ohmic contact layer. However, forming of the unalloy ohmic contact layer is difficult to control, and the metal layer, the dielectric layer and the non-ally ohmic contact layer are all required to fully utilize the good reflectivity of the metal layer.